10 INTRODUCTION. 



wood-charcoal, where more than a third of the quantity of 

 gas is condensed in a liquid form on the walls of the cells) ; 

 and the chemical action of " contact substances/' which by 

 their presence (catalytically) occasion or destroy combina- 

 tions without taking themselves any part therein, all these 

 phsenomena teach that substances exercise upon each other, 

 at infinitely small distances, an attraction dependent on 

 their specific essences. Such attractions cannot be con- 

 ceived without motion excited by them, although escaping 

 our visual perception. In what relation this mutual mole- 

 cular attraction, viewed as a cause of perpetual motion 

 on the surface of the globe, and, it is highly probable, also 

 in its interior, may stand to the attraction of gravitation, by 

 virtue of which the planets, and the central bodies around 

 which they revolve, are in perpetual motion, is wholly 

 unknown to us. Even & partial solution of such a purely 

 physical problem would constitute the highest and most 

 glorious prize, which the combination of experiment with 

 intellectual reasoning could attain in such lines of research. 

 In the above allusions to molecular, and what is commonly 

 called Newtonian attraction, I have not been willing to 

 employ the latter term to designate exclusively the attraction 

 which prevails in the regions of space, extending to illimitable 

 distances, and acting inversely as the square of the distance. 

 Such an application of the word Newtonian appears to me 

 almost an injustice to the memory of that great man, who 

 already recognised both the manifestations of force, whilst 

 at the same time, as if anticipating future discoveries, he 

 attempted, in his appendix to his work on Optics, to at- 

 tribute capillarity, and the little that was then known of 

 chemical affinity, to universal gravitation (Laplace, Expos. 



